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The procedure is the same on iOS 10 (tested with 10.0.2), iOS 9 (tested with 9.3.5 and 9.2) and iOS 8 (tested with 8.1.2 and 8.4.1). Go to Settings, then Wi-Fi, then click/press on the name of the wireless network you are connected to. The IP address of the router is identified as "Router" in the DHCP section.
On iOS 7 (tested with 7.1.1), go to settings, then Wi-Fi, then click on the name of the wireless network you are connected to. As with iOS 8, the routers IP address is identified as "Router" in the DHCP section.
On iOS 6, go to settings, then Wi-Fi, then click on the arrow to the right of the network you are connected to. The interface is similar to that shown above, the router IP address is identified as "Router".
The instructions for iOS version 5 are identical to those for version 6.
All the instructions above assume that the iOS device was assigned its IP address using DHCP. In the rare case where it has a static IP address, click on the Static tab (shown above) and, again, look for "Router".